I'm confused by your question. Are you talking about hardware drivers (NI-DQ, NI-GPIB) or instrument drivers? You should be able to use current hardware drivers with older version of LabVIEW and you can (in most cases) use older hardware drivers with new versions of LabVIEW. For instrument drivers, NI only maintains LabVIEW 6 and 7 versions on it's Instrument Driver Network but I'm pretty sure maintains an archive of older versions if they exist. If you have a specific instrument driver that you need, make a request on this forum. NI might be able to provide an older version or someone can save it to a previous version. One or two probably won't be a problem but numerous requests might be a little out of line. At some point, you might want to consider a
n upgrade. There have been numerous improvements since 5.1 and upgrading is usually not a major problem.